There is a wide variety of different wireless communication networks. A few examples of modern networks include Global System for Mobile Communications (GSM), Wideband Code Division Multiple Access/High Speed Packet Access (WCDMA/HSPA), and Long Term Evolution (LTE), etc. The 3rd-Generation Partnership Project (3GPP) is continuing development of the LTE network technologies. Improved support for heterogeneous network operations is part of the ongoing specification of 3GPP LTE Release 11 (Rel-11), and further improvements are being discussed in the context of new features for Rel-11. In heterogeneous networks, a mixture of cells of different sizes and overlapping coverage areas are deployed.
One example of such a deployment is seen in a system where several pico-cells, each comprising a base station or low-power transmitting/receiving node with a respective coverage area, are deployed within the larger coverage area of a macro-cell, which comprises a base station or high-power transmitting/receiving node. As will be discussed in further detail below, the large difference in output transmit power, e.g., 46 dBm in macro-cells and 30 dBm or less in pico-cells, results in different interference scenarios from those that are seen in networks where all base stations have the same output transmit power.
Throughout this document, nodes or points in a network are often referred to as being of a certain type, e.g., a “macro-” node, or a “pico-” point. However, unless explicitly stated otherwise, this should not be interpreted as an absolute quantification of the role of the node or point in the network but rather as a convenient way of discussing the roles of different nodes or points relative to one another. Thus, a discussion about macro- and pico-cells could just as well be applicable to the interaction between micro-cells and femto-cells, for example.
One aim of deploying low-power nodes such as pico base stations within the macro coverage area is to improve system capacity, by means of cell-splitting gains. In addition to improving overall system capacity, this approach also allows users to be provided with a wide-area experience of very-high-speed data access, throughout the network. Heterogeneous deployments are in particular effective to cover traffic hotspots, i.e., small geographical areas with high user densities. These areas can be served by pico-cells, for example, as an alternative deployment to a denser macro network.
The most basic means to operate heterogeneous networks is to apply frequency separation between the different, so-called layers. For instance, the macro-cell and pico-cells can be configured to operate on different, non-overlapping carrier frequencies, thus avoiding any interference between the carrier frequencies, e.g. layers. With no macro-cell interference towards the under-laid cells, i.e., the cells having coverage areas falling substantially or entirely within the coverage area of the macro-cell, cell-splitting gains are achieved when all resources can simultaneously be used by the under-laid cells.
One drawback of operating layers on different carrier frequencies is that it may lead to inefficiencies in resource utilization and energy consumption. For example, if there is a low level of activity in the pico-cells, it could be more efficient to use all carrier frequencies in the macro-cell, and then basically switch off the pico-cells. However, the split of carrier frequencies across layers in this basic configuration is typically done in a static manner.
Another approach to operating a heterogeneous network is to share radio resources between layers (interpreted as carrier frequencies). Thus, two or more layers can use the same carrier frequencies, by coordinating data and/or control transmissions across macro- and pico-cells. This type of coordination is referred to as inter-cell interference coordination (ICIC). With this approach, certain radio resources are allocated to the macro-cells for a given time period, whereas the remaining resources can be accessed by the under-laid cells without interference from the macro-cell. Depending on the traffic situations across the layers, this resource split can change over time to accommodate different traffic demands. In contrast to the earlier described static allocation of carrier frequencies, this way of sharing radio resources across layers can be made more or less dynamic depending on the implementation of the interface between the nodes. In LTE, for example, an X2 interface has been specified in order to exchange different types of information between base station nodes, for coordination of resources. One example of such information exchange is that a base station can inform other base stations that it will reduce transmit power on certain resources.
Time synchronization between base station nodes is generally required to ensure that ICIC across layers will work efficiently in heterogeneous networks. This is of particular importance for time-domain-based ICIC schemes, where resources are shared in time on the same carrier.
Before an LTE terminal can communicate with an LTE network it first has to find and acquire synchronization to a cell within the network, a process known as cell search. Next, the user equipment (UE) has to receive and decode system information needed to communicate with and operate properly within the cell. Finally, the UE can access the cell by means of the so-called random-access procedure.
In order to support mobility, a terminal needs to continuously search for, synchronize to, and estimate the reception quality of both its serving cell and neighbor cells. The reception quality of the neighbor cells, in relation to the reception quality of the current cell, is then evaluated in order to determine whether a handover, for terminals in connected mode, or cell re-selection, for terminals in idle mode, should be carried out. For terminals in connected mode, the handover decision is taken by the network, based on measurement reports provided by the terminals. Examples of such reports are reference signal received power (RSRP) and reference signal received quality (RSRQ). Alternatively, a network-centric approach may be used, wherein the network performs measurements such as RSRP and RSRQ on uplink signals transmitted by the UE.
The results of these measurements, which are possibly complemented by a configurable offset, can be used in several ways. The UE can, for example, be connected to the cell with the strongest received power. Alternatively, the UE can be assigned to the cell with the best (i.e. largest) path gain. An approach somewhere between these alternatives may be used.
These selection strategies do not always result in the same selected cell for any given set of circumstances, since the base station output powers of cells of different type are different. This is sometimes referred to as link imbalance. For example, the output power of a pico base station or a relay node is often on the order of 30 dBm (1 watt) or less, while a macro base station can have an output power of 46 dBm (40 watts). Consequently, even in the proximity of the pico-cell, the downlink signal strength from the macro-cell can be larger than that of the pico-cell. From a downlink perspective, it is often better to select a cell based on downlink received power, whereas from an uplink perspective, it would be better to select a cell based on the path loss.
From a system perspective, it might often be better, in the above scenario, for a given UE to connect to the pico-cell even under some circumstances where the downlink from macro-cell is significantly stronger than the pico-cell downlink. However, ICIC across layers will be needed when the terminal operates within the region between the uplink and downlink borders, i.e., the link imbalance zone.
The concept of a “point” is heavily used in conjunction with techniques for coordinated multipoint (CoMP). In this context, a point corresponds to a set of antennas covering essentially the same geographical area in a similar manner. One transmitting/receiving node, such as an LTE base station, might control one or several points. Thus, a point might correspond to one of the sectors at a base station site, but it may also correspond to a site having one or more antennas all intending to cover a similar geographical area. Often, different points represent different sites. Antennas correspond to different points when they are sufficiently geographically separated and/or have antenna diagrams pointing in sufficiently different directions. Techniques for CoMP entail introducing dependencies in the scheduling or transmission/reception among different points, in contrast to conventional cellular systems where a point is operated more or less independently from the other points, from a scheduling point of view.
When downlink (DL) CoMP is applied, the network needs to dynamically or semi-statically determine which transmission points are to serve each UE in the DL. Additionally, the network needs to determine a set of points for which receiving feedback from the UE would be beneficial. Such a set of points for feedback reception is typically selected in a semi-static fashion (i.e., they are typically constant for several subframes) and the corresponding feedback may be employed for scheduling, link adaptation and dynamic selection of the transmission points within the set of points for which feedback is available. The set of suitable transmission points for a UE typically changes dynamically, e.g. as the UE moves through the network. The network therefore needs to select, and continuously update, a set of candidate transmission points for the UE. The UE then sends more detailed feedback, e.g. pre-coding information, for the points in the candidate set, thereby enabling the network to select the best downlink transmission points. The techniques mentioned above will be collectively referred to as “point selection” in the following.
The points in the candidate set may be determined in a UE-centric manner, wherein the UE performs measurements on downlink signals (e.g. reference signals provided for generating channel state information (CSI-RS), see also the appendix for a more extensive description) and reports the results to the network. Alternatively, a network-centric approach may be used for point selection, wherein the network performs measurements, e.g. pathloss, on uplink signals transmitted by the UE. For example, sounding reference signals (SRS) may be used for this purpose. A description of SRS and other reference signals (RS) can be found in the appendix.
Uplink (UL) power control (PC) for SRS is currently based on UL PC for physical uplink shared channel (PUSCH), with the exception of a power offset parameter (see also the appendix for a more extensive description). Typically, SRS are power controlled in order to reach the DL transmission point(s) in a Time Division Duplex (TDD) network, in case channel reciprocity is exploited, and the UL reception point for link adaptation, in case of both Frequency Division Duplex (FDD) and TDD.
On the other hand, in order to enable network-centric CoMP points selection and/or mobility measurements, SRS need to be received with sufficient quality at all points that are potentially involved in the CoMP operation. Such a set is likely larger than the set of points exploited for actual DL and/or UL CoMP operations. Such a mismatch may result in difficulty, or even impossibility, of estimating pathloss for certain UEs that are poorly received at nodes potentially suitable for DL CoMP transmission.
One possible solution would be to increase SRS power. However, this would result in increased interference as well as increased energy consumption for the UEs.
Another possible solution would consist of increasing the size of Downlink Control Information (DCI) formats to include independent closed loop (CL) PC bits for SRS and PUSCH or physical uplink control channel (PUCCH). However, such a solution has the undesirable drawback of increasing the signaling overhead, resulting in problems in terms of backwards compatibility as well as in reduced coverage and capacity for control channels.
Thus, it would be desirable to provide improvements related to uplink reference signals and network-centric measurements in a wireless communication network, for example estimation of received UE energy, such as pathloss estimation, or estimation of RSRP and/or RSRQ which can then be used for channel estimation. Such a mechanism would be beneficial e.g. for downlink transmission point selection and/or mobility and DL/UL link adaptation purposes. It would be particularly advantageous to be able to increase the estimation reliability and/or accuracy, while at the same time limiting the increase in interference and UE power consumption.